How can I create an orthographic render similar to the Portal 2 promotional videos?
It's not clear on what aspects your question is focussed. For this orthographic render I used BI since freestyle isn't available when using cycles. The caracters were made from a plane with a few extrusions and a circle.



Enable freestyle in the render tab :



To control which objects get outline,  create a group (here desk and sitting guy) after that go to the Render Layers Tab select the group in the highlighted box (red).



If you want to use cycles you might be interested in blender-cycles-toon-material-node-setup